Process for nondestructively testing with a desensitized silver halide radiographic layer

ABSTRACT

A method is disclosed of nondestructively testing an article for substantially invisible voids adjacent its surface. A radioactive gas is brought into contact with the surface of the article to be tested. It is retained by adsorption on those surfaces presented by the substantially invisible voids and at other surface locations. A silver halide radiographic layer is juxtaposed adjacent the surface being tested, and the location of the radioactive gas adsorbed within the substantially invisible voids is preferentially recorded within the silver halide radiographic layer.

United States Patent Gibbons et al.

i 1 Nov. 25, 1975 3,299,269 1/1967 Hanson et 250/304 OTHER PUBLICATIONS Arthur, Abstract of Application Ser.

published Feb. 26, 1952.

[ PROCESS FOR NONDESTRUCTIVELY TESTING WITH A DESENSITIZED SILVER HALIDE RADIOGRAPHIC LAYER (75] Inventors: Carl B. Gibbons, Columbus, Ohio;

Martha E Sewell, Rochester; Robert Taberv Webster both of Primary ExaminerArchie R. Borchelt Attorney, Agent, or Firm-C. 0. Thomas [73] Assignee: Eastman Kodak Company,

Rochester, NY.

Feb. 26, 1974 [57] ABSTRACT A method is disclosed of nondestructively testing an article for substantially invisible voids adjacent its sur [22] Filed:

[2H Appl- N05 446,050 face. A radioactive gas is brought into contact with the surface of the article to be tested. It is retained by 52 5 250/303; 250/304 adsorption on those surfaces presented by the substan L 1 2 5/02 tially invisible voids and at other surface locations. A 304, 364 silver halide radiographic layer is juxtaposed adjacent [58] Field of Search 250/302, 303

the surface being tested, and the location of the radio active gas adsorbed within the substantially invisible voids is preferentially recorded within the silver halide radiographic layer.

[56] References Cited UNITED STATES PATENTS 3,179,806 4/!965 Dixon et al. 250/303 20 Claims, 2 ng ig r US. Patent Nov. 25, 1975 3,922,545

PROCESS FOR NONDESTRUCTIVELY TESTING WITH A DESENSITIZED SILVER HALIDE RADIOGRAPHIC LAYER This invention relates to an improvement in a radiographic process in which adsorbed radioactive gas is used to locate substantially invisible voids in an article adjacent its surface. More specifically, this invention relates to such a process in which the location of that portion of the radioactive gas adsorbed within the substantially invisible voids is preferentially detected and recorded.

Eddy US. Pat. No. 3,62l,252, issued Nov. 16, 197i teaches a process for nondestructively testing an object for surface defects by selectively adsorbing radioactive krypton gas within any defects present. The radiation emitted by the adsorbed gas can be detected and recorded in a variety of ways to determine the presence of surface defects and, if present, their size and loca tion. The Eddy process is disclosed to be an improvement over prior radioactive gas sorption techniques, since it relies primarily upon adsorption rather than absorption, thereby producing a sharper definition of the surface defect.

Eddy has observed that adsorbed radioactive krypton gas is less readily desorbed from surface defects than from the remaining surfaces presented by the article to be tested. It is this differential in desorption that is relied upon by Eddy to produce a differential in radiation levels indicative of the location of the surface defects.

Eddy is correct in his observation that surface defects are more tenacious in their retention of adsorbed radioactive gas, where the article is of relatively uniform composition, such as the typical metal part being tested. However, in actual practice it has been noted that some materials readily sorb radioactive gas and will tenaciously retain this gas in a manner comparable to the adsorption by surface defects. Thus, with the Eddy process considerable difficulty would be experienced in attempting to locate surface defects in a metal part encased in a plastic sheath capable of sorbing and retaining radioactive krypton gas. A more common disadvantage associated with the Eddy process is that articles to be tested of nominally uniform composition tend to collect on their surfaces materials such as dust, salts, oils and the like, as the result of casual atmospheric exposure and handling. These surface deposits can serve as sorption sites on the article surface and can result in false indications of surface defects. Alternately, where an article is being tested in a fairly dirty condition, such generalized surface sorption of the radioactive gas can occur that the location of surface defects can be obscured by the high level of background radiation. The Eddy process then exhibits significant limitations in terms of required cleanliness and direct handling of articles to be tested as well as the protective coating or packaging materials that can be employed with the articles at the time of testing.

It is an object of this invention to improve upon the Eddy process, and, more specifically, it is an object to provide such process which is more selective and accurate in detecting and recording the location of surface adjacent voids and which places fewer limitations on the composition, surface preparation and handling of the article to be tested.

It is a more specific object to provide such a process which is capable of selectively producing a permanent, enlarged radiographic image of surface adjacent voids 2 even in the presence of ambient lighting. It is a further object to provide such a radiographic image directly on the article surface being tested.

These and other objects of this invention can be achieved, in one aspect, by providing a method of nondestructively testing an article for substantially invisible voids adjacent its surface. A radioactive gas is brought into contact with the surface of the article being tested and retained by adsorption on those surfaces presented by the substantially invisible voids and at other surface locations. A silver halide radiographic layer is juxtaposed adjacent the surface being tested, and the location of the radioactive gas adsorbed within the substantially invisible voids is preferentially recorded within the silver halide radiographic layer.

This invention can be better appreciated by reference to the following detailed description considered in conjunction with the drawings, in which FIGS. 1 and 2 are sectional schematic diagrams of an article being tested with dimensional relationships being modified for ease of illustration.

The present invention relates to nondestructive testing using radioactive gases. While this invention is generally applicable to the use of radioactive gases, we prefer to employ those radioactive gases which exhibit an extended half-life-i.e., more than a day-since this avoids the necessity of forming the gas immediately before use and further avoids high initial radiation levels. The radioactive gas can emit alpha, beta and/or gamma radiation. We prefer to employ radioactive gases which emit radiation that is readily attenuated in the atmosphere, such as beta radiation, so that the need for shielding is minimized. We further prefer to use radioactive gases that are relatively inert chemically.

Radioactive krypton-e.g., the radioactive isotope krypton---is particularly well suited for use in the practice of this invention. This particular gas is well suited for use in the present invention because of its relatively small atomic diameter, 3 angstroms, a 10 year half-life, and emission of 662 Kev beta and 517 KeV gamma radiation. This particular radioisotope has a branching ratio of 0.46 percent, that is, it emits 4.6 gammas for each 1,000 betas. Moreover, of the radioactive inert gases, it is readily available. lts inertness is a particularly desirable quality both from the standpoint of nondestructive testing and health, since it does not readily react chemically so as to be chemically bound to tissue or on materials to which it is adsorbed.

The present invention is applicable to the nondestructive testing of articles or article portions for surface adjacent voids. The term surface adjacent voids" is defined to include any spatial vacancy, such as a crack, hole, fissure, discontinuity, pore or the like, which extends beneath the surface of the article and which is at least partially penetrable from the surface by the radioactive gas. The surface adjacent voids typically exhibit large surface areas for radioactive gas adsorption in relation to their volumes. In the case of cracks the voids also typically exhibit depths which greatly exceed their widths. Typical detectable voids can open progressively toward the surface, as in an open crack, or can be wholely or partially occluded at its interface with the surface or internally, as in a closed crack. some detectable surface adjacent voids can take the form of pores which lie entirely beneath the surface.

The diversity of geometrical configurations is schematically illustrated in FIG. 1, in which article portion 100 is illustrated bearing a surface coating 102, which can, in fact, represent a series of surface treatments and coatings. A pore 104 is shown to lie in the article portion beneath its surface, but within sufficient proximity to permit radioactive gas diffusion into the pore. A crack 106 is shown in the article portion and extending to its surface, but entirely occluded by the surface coating. An open crack 108 also lies in the article portion. The crack 108 opens progressively toward the surface of the article portion. Further, a discontinuity 110 is present in the surface coating overlying the crack, so that direct communication exists between the crack and the surface.

It is readily apparent that neither the crack 106 nor the pore 104 can be detected by surface inspection, even under microscopic examination. Further, the open crack 108 can be of such small size that it is substantially invisible to the naked eye. While in theory the present invention is applicable to the detection of surface adjacent voids of both large and small size, in practice this process is particularly advantageous for the detection of surface adjacent voids which are substan tially invisiblethat is, those that are not readily detected by the naked eye upon purposeful scrutiny. Our process is capable of detecting surface adjacent voids of exceptionally small size and is limited only by the molecular dimensions of the radioactive gas employed.

For example, in using radioactive krypton it is theoretically possible to cause adsorption which provides detectable surface concentrations in voids of about angstroms in width and larger. Krypton-85, however, is usually supplied as a mixture of 5 percent krypton-85 in krypton-84, and thus, the theoretical sensitivity can be approached only by using pure krypton-85. Nevertheless, with the readily commercially available mixtures of krypton-84 and krypton-85, surface adjacent voids of only a few nanometers in width can be detected.

The article or article portions to be tested for surface adjacent voids according to our process can be formed of any material on which gas can be adsorbed and desorbed. In order to avoid residual radioactivity we prefer not to test articles or article portions with our process which permanently absorb large quantities of radioactive gas. However, it is recognized that most materials adsorbing radioactive gas will to some extent also absorb the gas. it is preferred to utilize materials wherein absorption is small as compared to adsorption. This is recognized to be a function not only of the material chosen for testing, but also the procedures employed in bringing the material and gas together. We have found our process to be particularly well suited to the testing of structural metals. We specifically contemplate the applicability of our process to ferrous metals, carbon, copper and alloys thereof, noble metals and refractory metals. The nondestructive testing of insulative inorganic materials, such as glass and ceramic materials is also contemplated. Our process is particularly well suited for the nondestructive testing of articles of all sizes and geometries that can be readily inspected on one or more surface portions by eye or with the aid of optical instruments. Our process is particularly applicable to materials that provide intricate or irregular shapes and which exhibit surfaces of high curvature radius.

Any conventional method for adsorbing radioactive gas on the surface to be tested can be employed in the practice of this invention. One preferred approach is to first remove ambient gas which may be adsorbed on the surface to be tested. This can be achieved by reducing the ambient pressure adjacent this surface, as, for ex ample, by placing the article to be tested in a vacuum chamber. After desorption of ambient gas, it is merely necessary to bring the radioactive gas into contact with the surface in order to achieve adsorption. Where the article surface is of uniform composition and is uniformly physically acted upon, the radioactive gas is adsorbed on the exposed surfaces relatively uniformly. It is recognized that the use of moderate pressures can have the effect of removing ambient gas and achieving adsorption of radioactive gas in a single step. Pressures of 1 or more atmospheres above ambient achieve radioactive gas adsorption quite readily. Adsorption of radioactive gas can also be facilitated by employing moderately elevated temperatures, typically less than about 30C. It is generally preferred not to employ significantly elevated temperatures or pressures, since absorption of radioactive gases is increasingly favored with increases in these parameters. [on bombardment of the surface to be tested is also useful in dislodging nonradioactive gas which is initially adsorbed.

The next step is to desorb radioactive gas to the extent possible from those surfaces of the article being tested other than those surfaces presented by the surface adjacent voids. This is undertaken by allowing the radioactive gas to diffuse. Within a few minutes disproportionation of concentration levels of adsorbed gas occurs. This is because the article surface typically presents an open diffusion path for the adsorbed gas while the surface adjacent void, because of its small size, offers a considerably more restricted diffusion path for adsorbed gas. Further, the surface to subtended area ratio of random voids and particularly cracks tends to be much higher than that of purposefully formed surfaces. Still further, the surface adjacent void is shielded from convection currents that promote desorption.

While Eddy relies entirely upon this differential desorption to locate the surface adjacent voids, we have discovered that in many instances materials are present on the surface of the article being tested which readily trap, but do not readily release, the radioactive gas. These materials are typically surface contaminants, such as dust that has settled on the article from the ambient atmosphere or salts or oils that have been picked up by the article in handling or prior processing. Frequently their presence or distribution is both unknown and unsuspected by the tester. Whereas adsorbed radioactive gas is selectively desorbed from article surfaces other than those presented by the surface adjacent voids, the radioactive gas associated with the surface contaminants tends to be tenaciously retained. Thus, after a period of time sufficient to merit selective de sorption of surface adsorbed radioactive gas, the gas will still remain both at locations of adsorption by surface adjacent defects and at sites of surface contaminants.

This is illustrated diagrammatically in FIG. I. As shown in this figure the article portion with its surface coating 102 has been exposed to radioactive gas. The article portion is shown to have as surface adjacent voids sought to be detected, pore 104, occluded crack 106 and open crack 108. A surface contaminant 112 is also located on the surface coating. When the article is initially exposed to radioactive gas, molecules of the gas are adsorbed substantially uniformly on the sur faces presented by the article. The radioactive gas also diffuses into the article to some extent. For example, the radioactive gas diffuses through the surface coating to reach the crack 106 and diffuses through both the surface coating and the underlying article portion to reach the pore 104. At the same time the radioactive gas can be adsorbed or absorbed (most typically the latter) at the surface contaminant 112. After the article portion is removed from the radioactive gas atmosphere in which sorption occurs, that portion of the radioactive gas sorbed at the surfaces, other than those presented by the surface adjacent voids and the contaminant, selectively diffuse away from the article surface under examination, since they are comparatively weakly associated and by their location freer to migrate. This leaves the residual pattern of radioactive gas molecules schematically shown by circles 114.

At the stage of processing shown in FIG. 1 it is possi ble to detect the radiation given off by the sorbed gas using any one of the various detection techniques taught by Eddy or taught by Gibbons in copending, commonly assigned patent application Ser. No. 432,019, filed Jan. 9, 1974. Unfortunately, if these radiation-detecting techniques are employed, the tester will record a signal from the surface contaminant 112 and thereby mistake it for a surface adjacent void. If no surface adjacent voids were present, it can be appreciated that this could result in the article being rejected merely because it is dirty rather than because it is defective. On the other hand, if the surface contaminant 112 were of such lateral extent that it overlay the surface adjacent voids 104, 106 and 108, this could result in the signal from these surface adjacent voids being obscured by the surface contaminant. In either instance the tester can be misled if he relies upon the test. At best the test must be repeated and at worst the objective of the test may not be achieved, but without this being realized.

We have discovered quite unexpectedly that by juxtaposing a properly chosen silver halide radiographic element adjacent the surface being tested it is possible to record preferentially within its silver halide layer the location of the radioactive gas adsorbed within the surface adjacent voids as opposed to the location of the radioactive gas lying on the surface of the test article. In a simple, exemplary form a conventional radiographic element having a properly chosen silver halide layer can be employed in the practice of our process. The radiographic element is merely laid over the surface of the article to be tested after reaching the stage of processing shown in FIG. 1, and the radiation emitted by the radioactive gas is allowed to expose the radiographic element. Subsequently, after processing in a known manner, the radiographic element will show a visible record of the location of the surface adjacent voids and will magnify the size of the surface adjacent voids (attributable to radiation spreading) so that they can be more readily observed. However, no image will be produced by surface radiation, as, for example, that given off by radioactive gas sorbed by surface contaminants. In this way a visible record is obtained which accurately discriminates between surface contaminants and surface adjacent voids.

We have discovered quite unexpectedly that if a silver halide radiographic layer is employed which in cludes a properly chosen desensitizer for room light, it will also have the surprising effect of permitting discrimination between sites of surface adjacent voids and surface contaminants (or other sites of surface emissions). While we do not wish to be bound by any particular theory, we believe this discrimination is possible by the selective desensitization of the silver halide radiographic layer to high energy radiationthat is, to emissions having a potential in excess of 300,000 electronvolts (eV). By using a radioactive gas which emits principally high energy radiationas, for example, krypton gasthe radiation given off from the surface sites of the radioactive gas is principally of high energy to which the silver halide radiographic layer has been desensitized. On the other hand, high energy radiation given off from surface adjacent voids to a large extent impinges upon the surfaces of the article being tested. This is believed to stimulate secondary emissions of intermediate potentials e.g., from 20 to 200,000 eV. Thus, the radioactive gas adsorbed within the surface adjacent voids produces intermediate energy radiation which can be selectively detected and recorded in pref erence to room light (4 electron-volts) and high energy radiation. It is believed that the stimulated intermediate energy radiation is principally X-ray radiation.

We have found to be surprisingly effective for purposes of this invention the use of thiuram disufide silver halide desensitizers of the type disclosed in US. Pat.

No. 3,184,313, issued May 18, 1965. These compounds are characterized by marked desensitizing action toward silver halide radiographic layers insofar as their sensitivity to visible radiation is concerned (particularly in the blue region of the spectrum) while re taining substantially the sensitivity of the radiographic layers to X-rays. The concentration of thiuram disulfide silver halide desensitizer employed is subject to wide variation depending upon the particular silver halide used, the amount of gelatin or other colloidal binder and other variables. In general, quite useful results can be obtained at concentrations as low as 2 mg per mole of silver halide, while concentrations as high as mg per mole of silver halide can be used without adversely affecting the X-ray sensitivity too seriously. The preferred range of thiuram disulfide employed in the practice of this invention is in the range of about 20 to about 30 mg of thiuram disulfide per mole of silver halide. Typical thiuram disulfide desensitizers which can be employed in the practice of this invention include those represented by the following general formula:

wherein R and R each represents an alkyl group, such as methyl, ethyl, propyl, butyl isobutyl, etc. (e.g., an alkyl group containing from 1 to 4 carbon atoms), an aryl group, such as phenyl, tolyl, etc., or together R and R represent the atoms necessary to complete an azine ring, such as morpholine, piperidine, etc., or, alternatively, a pyrrolidine ring, for example. The thiuram disulfide compounds are characterized in that the nitrogen atoms thereof are tertiary nitrogen atoms. Typical compounds included by the above general formula are, for example, the following: bis(dimethylthiocarbamyl) disulfide, bis(diisobutylthiocarbamyl) disulfide, bis(4- morpholinothiocarbonyl) disulfide, bis(l-piperidylthiocarbonyl) disulfide, bis(diethylthiocarbamyl) disulfide, bis(n-methyl-N-phenyl-thiocarbamyl) disulfide,

7 bis(diphenylthiocarbamyl) disulfide, etc.

We prefer to employ in combination with the thiuram disulfide desensitizer a piazine, as taught in U.S. Pat. No. 3,403,025, issued Sept. 24, I968. While thiuram disulfide desensitizers are known to be effective in desensitizing dry silver halide radiographic layers to light, piazines in combination further desensitize silver halide radiographic layers to light during wet processing. The piazines we prefer to employ are heterocyclic compounds having two nitrogen atoms in the para position. The piazines contain a pyrazine ring to which two sixmembered carbocyclic rings are fused symmetrically and can be in the free or quaternized form. Where reference is made in the specification and claims to a piazine containing a pyrazine ring to which two sixmembered carbocyclic rings are fused symmetrically, it is intended to include the compound in its free or quaternized form. As shown in the formulas which follow, the pyrazine ring in the piazine is fused to two symmetrical carbocyclic rings which can be unsubstituted, as in phenazine, or substituted by a variety of substituents including one or more other carboxyclic rings, for example, one or two fused aromatic rings as in naphthophenazine and naphthazine. Examples of substituents which can be present to form the resulting substituted phenazines or naphthazines include amino, hydroxy, methyl amino, ethyl amino, methyl, ethyl, dodecyl, carboxyethyl, sulfopropyl, sulfobutyl, phenyl, benzyl, acetyl, and the like. It is obvious that the nature of the substituents on the two six-membered fused rings in the piazine is subject to wide variation. However, it can be seen that these piazines are characterized by a pyrazine ring to which two six-membered carbocyclic rings are fused symmetrically.

Piazines having up to about 24 carbon atoms, preferably 12 to 20 carbon atoms, give particularly advantageous results in the practice of this invention. Piazines which are particularly useful in the practice of this invention can be represented by the following formulas:

R f l Gra n N where each of R and R is amino, hydroxy, hydrocarbyl amino, alkyl, aryl, acyloxy, acylamino or a fused sixmembered carbocyclic ring and each n is an integer of to 2, or

where each of R and R is amino, hydroxy, hydrocarbyl amino, alkyl, aryl, acyloxy, acylamino or a fused sixmembered carbocyclic ring; R is alkyl, hydroxyalkyl or aryl; X is an anion such as halogen, chlorate, sulfate, methyl sulfate, p-toluene sulfate and the like; each n is an integer off) to 2 and z is an integer of!) to 2. The piazines containing other rings which are fused to the sixmembered carbocyclic rings which are in turn fused to the pyrazine ring can also be represented by the formulas:

i 1 N 1 jb where each of a, b, c and d is an integer of 0 to l and each Z represents the non-metallic atoms necessary to form a six-membered carbocyclic ring, and

where R, X, Z, a, b, c, d and Z are as defined above.

Like the thiuram disulfides, the concentration of piazine employed in the practice of this invention is subject to wide variation. in general, however, quite useful results can be obtained with concentrations in the range of about 1 to about 50, preferably about 2 to about 10 mg per mole of silver halide. It is preferred that the desensitizing combination contain a minor portion of the piazine in comparison to the thiuram disulfide. Generally useful results can be obtained with combinations in which the weight ratio of thiuram disulfide to piazine is in the range of about 2zl to about 20:1, preferably about 2:1 to about 10:1. Typical piazines which can be employed in the practice of this invention include the following: phenazine, Z-hydroxyphenazine, 3-amino-2-hydroxyphenazine, 2,3-diaminophenazine, 3,6-diaminophenazine, S-amino-l-dimethylaminophenazine, Z-methyl-3-amino-7-dimethylaminophenazine, naphthophenazine, naphthazine, phenanthrophenazine, 2,3-dihydroxyphenazine, 2-acetamino-3- acetoxyphenazine, l,3-diamino-S-methylphenazinium chloride, l,3diamino-iphenylphenazinium chloride, 3,7-diamino-5-phenylphenazinium chloride, 2,8- dimethyl-3,7-diamino-5-phenylphenazinium chloride, 3-amino-7-phenylamino-S-phenylphenazinium chloride, 2,3,7,8-phenylamino-5-phenylphenazinium chloride, l,3-diamino-5-ethylphenazinium chloride, etc. It is possible to employ the piazines alone as well as in combination with thiuram disulfides as desensitisers to high energy, surface radiation.

Instead of employing a thiuram disulfide and/or a piazine as a desensitizer for high energy surface radiation, desensitizers can be employed of the type disclosed in U.S. Pat. No. 2,541,472, issued Feb. 13, ll. These desensitizers can take the form of benzothiazole, quinoline, indolenine, benzotriazole, and rhodanine compounds having one or more nitro groups attached to a benzene nucleus which is either fused with the heterocyclic nucleus or is attached to it through a doubly bonded carbon-to-carbon chain (i.e., a methine chain linkage). The quaternary salts of the benzothiazolc, quinoline and indolenine compounds are also suitable. Exemplary suitable compounds include 2-(o-nitrostyryl)-3-ethylbenzoylthiazolium iodide, as well as corresponding mand p-nitrostyryl compounds; 2-(o,pdinitrostyryl)-benzothiazole; 2-(p-nitrostyryl)-quinoline metho-p-toluenesulfonate, as well as correspond ing 0- and m-nitrostyryl compounds; 4-(p-nitrostyryl)- quinoline methiodide; 3,3-dimethyl-2-(p-nitrostyryl)- N02 S E CHI VII and R, and R each are an alkyl group, for example, methyl, ethyl, propyl, butyl and the like. Examples of preferred nitrothiazolinothiacyanines are:

wherein X is halide such as chloride, bromine, or iodide 3-methyl-3'-methyl-6'-nitrothiazolinothiacyanine iodide,

3-ethyl-3-ethyl-6'-nitrothiamlinothiacyanine iodide,

3-ethyl-3 -methyl-6'-nitrothiazolinothiacyanine iodide,

3-butyl-3 '-butyl-6'-nitrothiazolinothiacyanine chloride,

3-ethyl-3 '-ethyl-5 '-nitrothiazolinothiacyanine iodide,

3-ethyl-3 '-methyl-5 '-nitrothiazolinothiacyanine chloride,

3-methyl-3-methyl-6'-nitrothiazolinothiacyanine iodide and 3 -butyl-3 -butyl-5 '-nitrothiazolinothiacyanine chloride. These desensitizers are useful in concentrations of from 0.02 to 2.0 grams per mole of silver halide, preferably from 0.5 to 1.0 gram per mole.

[t is also possible to employ as high energy surface radiation desensitizers nitroindazoles in which the benzene ring is substituted with one or two nitro groups. Preferred nitroindasoles are S-nitroindazoles, such as S-nitroindazole and l-methyl-S-nitroinazole, as div closed in British Pat. No. 1,269,268 published Apr. 6, 1972 and U.S. Pat. No. 2,271,229, issued Jan. 27, 1942. These desensitizers are useful in concentrations of from 0.1 to 5 grams per mole of silver halide, preferably from I to 3 grams per mole.

Still another class of high energy surface radiation desensitizers useful in the practice of this invention 5 mercapto tetrazoles of the general tautomeric formula:

N T r where R is a hydrogen atom or a hydrocarbon substitutent group (which itself can be substituted or unsubstituted). Examples of such hydrocarbon groups are alkyl groups, e.g. methyl, ethyl and higher alkyl groups, and unsaturated groups, such as allyl; aryl groups, e.g. phenyl and naphthyl; aralkyl groups, e.g. benzyl; and cycloalkyl groups, e.g. cyclohexyl. Such groups can themselves be substituted, as by such groups as alkoxy, phenoxy, halogen, cyano, nitro, amino, substituted amino, sulfo, sulfamyl, substituted sulfamyl, sulfonylphenyl, sulfonylalkyl, fluorosulfonyl, sulfonamidophenyl, sulfonamidoalkyl, carboxylic acid, carboxylate, carbamyl, carbamylphenyl, carbamylalkyl, carbonylalkyl, carbonylphenyl and similar groups. Preferred 5- mercapto tetraaoles are l-phenyl-S-mercapto tetrazoles. Further exemplary of these are l-(aminophenyl)-5-mercapto tetrazoles, such as those of the structural formula:

VIII.

wherein M is a member of the class consisting of alkali metals and hydrogen and X is a radical having a formula selected from the group consisting of -SO,R' and COR, wherein R is a hydrocarbon or substituted hydrocarbon similar to R, defined above. Specific preferred exemplary compounds of this type include I- methyl-S-mercapto tetrazole; l-a-naphthol-5-mercapto tetraaole; i-cyclohexyl-S-mercapto tetraz ole', lphenyl-S-mercapto tetramole; l-( 3-acetamido )-phenyl- S-mercapto tetrazole; l-( 3-caproamido)phenyl-S-mercapto tetrazole; l-(3-pelargonamido)-phenyl-5-mercapto tetrazole; l-(B-methylsulfonamido)-phenyl-5- mercapto tetrazole, etc. These and similar S-mercapto tetrazoles are further disclosed in U.S. Pat. Nos. 2,403,927 and 3,295,976, issued July l6, 1946 and Jan. 3, I967, respectively. These S-rnercapto tetramles can be employed in concentrations of from 0.05 to 2 grams per mole of silver halide, preferably from 0.25 to 0.5 gram per mole.

In addition to the foregoing compounds it is believed that still other compounds can be employed usefully as desensitizers in the practice of our invention, such as nitron compounds of the type disclosed in West German Pat. No. 1,995,232, granted Nov. 3, I969, and U.S. Pat. No. 3,740,226, issued June 19, 1973.

It is apparent from the patent citations given that each of the deaensitizers noted above are known to those skilled in the photographic art, although the present utility was not recognized. It is accordingly apparent that these desensitizers, singly or in combination, can be incorporated into conventional silver halide radiographic layers by procedures well within the skill of the art and that no useful purpose would be served by redescn'bing the procedures for their preparation or incorporation herein. Further, in the foregoing description of the desensitizers, it is to be appreciated that references to alkyl moieties refer to those of 20 or fewer carbon atoms, preferably 6 or fewer carbon atoms, unless otherwise indicated; and that aryl moieties are preferably phenyl or naphthyl moieties, unless otherwise indicated.

Noting FIG. 1, it is apparent that the discontinuity 110 provides a possible excape passage for the radioactive gas lying within the open crack 108, although no ready escape path is available to the radioactive gas lying in the closed crack 106 or pore 104. According to Eddy it is necessary to move quickly from gas adsorption to surface examination in order to detect open surface defects, since the surface radiation count drops by half in 8 to 9 hours. The radiation drop is more significant with respect to larger surface voids sought to be located than for smaller surface voids. Since Eddy em ploys krypton as a radioactive gas, the decrease of the radiation count is almost entirely a function of the de sorption rather than its halfdife.

To guard against the diffusion of adsorbed radioactive gas from open surface voids we prefer to cover the surface of the article being tested with a sealant composition at the stage of processing shown in FIG. 1, that is, after selective adsorption of the radioactive gas has been accomplished. The sealant composition can be applied using any conventional coating technique.

We prefer to employ the chillsetting coating technique for sealant application disclosed by Gibbons in the above-noted, copending patent application. This can be readily and advantageously accomplished by re lating the temperatures of the article surface to be tested and the sealant composition so that the article surface is at a temperature below the chill set temperature of the sealant and the sealant composition is at a temperature above its chill set temperature. When the sealant composition and the article are then brought together a uniform temperature differential is present between all contacted surfaces of the article and the sealant composition. The portion of the sealant lying adjacent the article surfaces is then cooled at a uniform rate and as a function of its distance from the surface of the article. It is then apparent that isothermal zones will be generated which concentrically surround the article surface brought into contact with the sealant composition and that, as the sealant composition is cooled by the article, a zone corresponding to or below the chill set temperature of the sealant composition will irradi ate outwardly from the contacted article surfaces. Sealant lying within this zone forms a chill set coating of substantially uniform thickness. The thickness of the coating can be readily controlled as either a function of the initial article temperature, as a function of the duration of unset sealant composition-article contact, or by restricting the availability of sealant for setting. In a preferred form the sealant is rapidly, preferably substantially instantaneously, chill set on the article surface. This is achieved by adjusting the relative article and sealant composition temperatures so that the article is at a temperature of at least C. below the chill set temperature of the sealant. In most instances it is desired to cool the article to a temperature substantially below the chill set temperature of the sealant. This is typically at least C below the chill set tern perature. It is, of course, not essential that the article to be tested be cooled at all. By proper choice of sealant compositions it may be more convenient to heat the 12 sealant composition to a temperature above ambient and sufficiently above its chill set temperature to establish the desired temperature differential. The chill set temperature can be chosen so that it is above or below the ambient temperature.

A number of manipulative procedures can be em ployed for forming the chill set sealant coating. In a simple, preferred procedure the article is simply dipped into the sealant composition and removed when the desired coating thickness is achieved by chill setting. In an alternate technique the sealant composition can be sprayed onto the article surface to be coated until the desired coating thickness is obtained. In most instances the chill set sealant coating will be quite thin, typically less than a millimeter and most typically below about 500 microns. Such coating thicknesses are readily ob tained in a substantially instantaneous manner. If it is desired to build up sealant coatings of very substantial thickness this can usually be accomplished in a single coating step. in some instances it may be desired to form multiple coatings either for the purpose of altering the composition from layer to layer or to achieve greater thicknesses. For example, where the sealant composition is a poor thermal conductor, it may be desirable to first form a coating and then to allow the coating to reach thermal equilibrium with the article before forming an additional coatiing. In this manner very thick coatings can be formed while any tendency toward nonuniformity is minimized.

The sealant composition can be applied in the form of an emulsion, latex or dispersion, for example, incorporating the sealant. The sealant can take the form of any viscous or solidifiable material which will immobilize the radioactive gas. Natural and synthetic polymers are readily employed as sealants in combination with a solvent or liquid dispersant. While the sealant and its dispersant or solvent can be used in widely varied proportions, the sealant is typically employed in concentrations of from about 5 to percent by weight based on total weight. Instead of being diluted with a solvent or dispersant, the sealant can be used alone in a liquid state. For example, the sealant can be applied in a molten state and chill set on the article being tested. Exemplary preferred sealants include polyvinyl alcohol, polyvinylidene chloride, polyvinyl acetals (e.g., polyvinyl butyral), polyalkylenes (cg, polyethylene, poly-- propylene, etc.), polyhaloalkylenes (e.g., polyvinylidene fluoride) and more specifically polyperfluoroalkylenes (e.g., polytetrafluoroethylene, polyhexafluoropropylene), polyamides, collodion, cellulose, gelatin, etc.

The solvent or dispersant can take any convenient form, but is preferably chosen to be readily volatilized so as to leave behind a residual sealant coating. Volatilization is preferably accomplished under conditions of temperature and humidity that minimize surface hardening of the sealant so that large amounts of solvent or dispersant are not occluded by the sealant coating. Typical solvents and dispersants include water, tetrahydrofuran, alcohols (e.g., methanol, ethanol, propanol, etc.), ketones (cg, acetone, etc.), aldehydes (e.g., acetaldehyde, etc.), volatile hydrocarbons (e.g., butane, pentane, octane, etc.), amides (e.g., dimethylformamide, etc.), and other well known volatilizable solvents.

The sealant need not itself be capable of producing an image or alterable in any way by the radioactive gas; however, the sealants are not limited to nonradiationsensitive materials. Sealants can be employed which are capable of imaging or of alteration by radiation released by the trapped radioactive gas or from radiation from other sources. For example, a photoresist composition can be employed as a sealant which is permanently set in place after initial chill setting by exposure to light or other radiation. Such photoresists can be formed from any conventional photopolymerizable or photocrosslinkable material. Heat setting of sealants, as by heat induced crosslinking, after chill setting is specifically contemplated.

The sealant layer can itself selectively produce an image corresponding to the surface adjacent voids which have adsorbed the radioactive gas(as opposed to surface sorbed radioactive gas), provided the sealant is chosen to be a silver halide radiographic material which has been desensitized to high energy surface radiation or is employed in combination with such a material. Such silver halide radiographic materials offer the advantages of large image amplification common to conventional silver halide radiographic materials, all of the advantages of sealant compositions described by Gibbonss and, in combination, the advantages of surface radiation discrimination of our process.

Conventional silver halide radiographic emulsions and dispersions modified by the inclusion of high energy surface radiation desensitizers, described above, are specifically contemplated to be useful as sealants in the practice of this invention. Useful silver halides include silver chloride, silver bromide, silver chlorobromide, silver iodide, silver chloroiodide, silver bromoiodide and silver chlorobromoiodide. It is specifically preferred to employ silver bromoiodides because of their comparatively high speed. It is also preferred to use silver halide grains of comparatively large size, typically above 0.5 microns, since sneed increases with increasing grain size. The larger the grain size the greater the probability of the grain being struck by emitted radiation. Suitable large grain, fast emu sions and dispersions are conventionally employed in forming radiation-responsive elements for x-ray applications. The emulsions can be coarse or fine grain emulsions and can be prepared by a variety of techniques e.g., single jet emulsions, double jet emulsions, ammoniacal emulsions, etc.

The silver halide emulsions and dispersions can be sensitized with chemical sensitizers, such as with: reducing; sulfur, selenium or tellurium compounds; gol'l, platinum or palladium compounds; or combinations of these. Procedures for chemically sensitizing silver halide emulsions are described in Sheppard et a1. U.S. Pat. No. 1,623,499 issued Apr. 5, 1927; Waller et al. U.S. Pat. No. 2,399,083 issued Apr. 23, 1946; McVeigh U.S. Pat. No. 3,297,447 issued Jan. 10, 1967 and Dunn U.S. Pat. No. 3,297,446 issued Jan. 10, 1967.

Silver halide emulsions and dispersions can contain development modifiers that function as speed increasing compounds such as polyalkylene glycols, cationic surface active agents and thioethers or combinations of these as described in Piper U.S. pat. No. 2,886,437 issued May 12, 1959; Darin et al. U.S. Pat. No. 3,046,134 issued July 24, 1962; Carroll et al. U.S. Pat. No. 2,944,900 issued July 12, 1960 and Goffe U.S. Pat. No. 3,294,540 issued Dec. 27, 1966.

The silver halide emulsions and dispersions can be protected against the production of fog (that might be induced by the composition of the article surface being tested or by other materials) and can be stabilized 14 against loss of sensitivity during keeping. Antifoggants and stabilizers can be used alone or in combination.

Photographic silver halide sealant layers can contain various sealants alone or in combination as vehicles. Suitable hydrophilic vehicle materials include both naturally occurring substances such as proteins, for example, gelatin, gelatin derivatives, cellulose derivatives, polysaccharides such as dextran, gum arabic and the like; and synthetic polymeric substances such as water soluble polyvinyl compounds like poly(vinylpyrrolidone), acrylamide polymers and the like.

Photographic sealant layers can contain alone or in combination with hydrophilic, water-permeable colloids, other synthetic polymeric vehicle compounds such as dispersed vinyl compounds such as in latex form and particularly those which increase the dimensional stability of the photographic materials. Typical synthetic polymers include those described in Nottorf U.S. pat. No. 3,142,568 issued July 28, 1964; White U.S. pat. No. 3,193,386 issued July 6, 1965; Houck et al. U.S. Pat. No. 3,062,674 issued Nov. 6, 1962; Houck et al. U.S. Pat. No. 3,220,844 issued Nov. 30, 1965; Ream et al. U.S. Pat. No. 3,287,289 issued Nov. 22,

1966; and Dykstra U.S. Pat. No. 3,411,911 issued Nov. 19, 1968. Other vehicle materials include those waterinsoluble polymers of alkyl acrylates and methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylates, those which have cross-linking sites which facilitate hardening or curing as described in Smith U.S. Pat. No. 3,488,708 issued Jan. 6, 1970, and those having recurring sulfobetaine units as described in Dyl stra Canadian Pat. No. 774,054.

The photographic sealant layers can contain surfactants such as saponin; anionic compounds such as the alkyl aryl sulfonates described in Balds efen U.S. pat. No. 2,600,831 issued June 17, 1952; amphoteric compounds such as those described in Ben-Ezra U.S. Pat. No. 3,133,816 issued May 19, 1964; and water soluble adducts of glycidol and an alkyl phenol such as those described in 01in Mathieson British Pat. No. 1,022,878 issued Mar. 16, 1966; and Knox U.S. Pat. No. 3,514,293 issued May 26, 1970.

Since many surfaces offer poor visual contrast with the radiation-induced image, it is contemplated that a pigment or dye can be incorporated into the sealant layer. While the temperature of the article and the sealant composition can be related to control the thickness of the sealant coating, because of the desire for nearinstantaneous chill setting to occur, it can in some instances be desirable to build up coatings somewhat thicker than actually required for image-recording purposes. The use of pigmentsparticularly white pigments, such as titanium dioxide is a means of reducing the effective photographic thickness of a sealant coating. The titanium dioxi e effectively masks any background printout attributable to lack of fixation of the inner portions of the sealant layer.

In a number of applications it can be desirable to stir the sealant composition prior to coating. This can lead to the entrapment of air bubbles where mechanical stirring is employed. It has been discovered that the incorporation of a thickening material within the sealant composition can significantly reduce the entrapment of air bubbles. Exemplary of useful known thickening agents are hydroxyethylcellulose and polymeric sulfonate thickening agents of the type disclosed in copending U.S. patent application Ser. No. 239,389, filed Mar. 29, 1972. These thickening agents are preferably incor- 15 porated in concentrations of from 0.1 to percent by weight of the dispersion. Preferably a thickening agent and a surfactant are employed in combination for this purpose.

Where the sealant layer is not itself an imaging layer that is capable of forming a visible or latent image, the sealant layer acts to immobilize the adsorbed radioactive gas while a separate silver halide radiographic layer or element is employed to detect preferentially the radiat on emitted from the surface adjacent voids. In a simple form a conventional silver halide radiograph c element containing the high energy surface radiation desensitizer can be physically positioned over the nonimaging sealant coating and held in position for a length of time sufficient to achieve exposure. It is also contemplated that a second sealant layer, which in this instance can be identical to the desensitized silver haldie radiographic sealant layer described above, can overlie the first sealant layer. A subbing layer can be disposed between the two subbing layers. Subbing layers re well known in the photographic arts for facil ating t e coating of a hydroph ic radi tion-respon ve layer onto a hydrophobic su face. It is specifically contemplated that the first sealant layer can take the fo m of a conventional p o ographic subbing composi on. Instead of employing a separate subbing l yer, to first sealant can be chemically treated or mech nically textured to bond the second sealant layer. The first sea ant layer, where it is cho en to be nonimaging, can be ident cal in composit on to a silver halide emulsion or dispers on, but lack radiation-responsive silver halide. The first sealant layer, in addition to its sealant function, can also perform the valuable function of prot cting the second, radiation-responsive sealant layer from di rect physical contact with contaminants associated with the article surface. While two layers are di cussed above, it is recognized that the sealant layers can be further multiplied without detracting from their intended functions. It is preferred that all of th sea ant layers be chill set; however, this is not essential. In reost instances, at least the first applied sealant layer is c ill set according to the practice of this invention.

In many instances, a radiographic image will printout directly on the object being tested without any processing whatsoever. In other in stances, processing of a co ventional type will be required to produce a visible image that is not destroyed upon viewing in ambient lighting. In most instances, it will be convenient to process the object to be tested with radiation-responsive material located directly thereon. This avoids having to correlate the object with a radiographic record of any void, crack or discontinuity contained therein. In other instances, as where a desens tized radiographic element s employed for imaging, it may be convneient to remove the element for processing separate from the object being tested. In still other instances, it may be convenient to strip the sealant layer or a radiation-responsive imaging layer overlying the sealant layer (which, of course, may also be a sealant layer) from the object for the purposes of separate processing. Separate processing may be convenient where the composition of the object or some part thereof would tend to contaminate the processing materials being used for image development. The dispersion, after processing on the article, can be easily removed mechanically or chemically. For example, it is well known to strip silver halide gelatin coatings with alkali. To avoid chemical attack on the 16 article, a gelatinase enzyme as been found to be a quite effective stripping agent.

The practice of our process employing a high energy, s 'f ce radi 'ien desensi zed si ver halide radiograp ic layer as a sealant layer as well as a selective imaging layer is schematically illustrated in FIG. 2. In this figure the article at the stage of process ng shown in F'G. l is coated with a silver halide radiographic layer 116. The silver halide layer tends to restrain the radioactive gas a sorbed on the surface of t e open crack ."l so th t it is un ble to escape through the discontin ity 110. This stabilizes the adsorption pattern as of the time of co ting and prevents the larger, less occlu ed surface adjacent voids f om exh biting such a loss of adsorbed rad oactive g s as to reduce their likelihood of detect on.

S lver ha ide crystals 118 are initially more or less uni ormly dispersed within the radiographic sea ant layer, as is well un erstood by those skilled in the art. The sil er hal de crysta s in t ally overlie both the s rface a jacent voids 104, 106 ard 198 ard the surface c ct 'r ant 112, ll of wh ch cont in sorbed radioactive g s. In addition to the s lver h lide and a suitable dispersart therefor the radiographic layer additionally c ntains at least one of the high energy, surface radiat cn desensit zers above described. The radioactive gas associa ed with t e surface contaminant 11.2 sends off high energy emissions. These emissions, because of th ir surface position, are readily transported to the silver halide radiograp ic layer, which has been desensitired to them. Accordingly, few, if any, of the silver hali e p rticles d spersed in the radiograph c layer ove yirg the contaminant 112 are converted to met llic silver and no vis b e silver ma ks are produced.

On the other hand, radiation given off by the adsorbed radioactive gas associated with the surface adjacent voids is more likely to undergo one or more collisions with the atoms of the article being tested. In doing so, it is believed that these emissions from the radioactive gas stimulate secondary emissions on impact with the a oms of the test article. These secondary emissions are believed to be of intermediate energy levels. As shown in FIG. 9, they produce a visible or latent image in the silver halide crystals overlying and extending latora ly somewh t beyo d the su face adjacent voids. The image forming silver halide crystals are indi ated in FIG. 2 as so id triangles 17 The following specific embodiments are set forth to further illustrate the practice of this invention:

Ges containing krypton-35 was introduced into surface imperfectio s of jet eng ee turbine bl des by the following means: The blades were placed in a vacuum system (commercially available as 21 Norton Vacuum System, Model 31 17) which is capab e of pumping below 10 torr. After the pressure of the bell jar of the vacuum system was reduced below l0 torr., the pumping system was sealed off from the bell jar and a gas containing kryptonwas bled in until the pressure was only slightly less than atmospheric pressure. A high speed pumping system was the used to remove the krypton-85 gas. The pump was sealed off and the inside of the bell jar was quickly brought to atmospheric pressure by opening it to room air. The turbine blades were immediately removed from the bell jar and coated with approximately 25 microns (dry thickness) of polyvinyl alcohol by the following method: A 5 percent solution of polyvinyl alcohol in water was applied to the turbine blade by dipping the turbine blade into the sealant composition. The turbine blade was cooled to approximately C in liquid nitrogen prior to dipping. The turbine blades were dipped and removed in a continuous manual movement. The polyvinyl alcohol instantaneously chill set as a substantially uniform coating on all surfaces of the turbine blades brought into contact with the sealant composition. The coating was noted to extend around the edges of the turbine blades wit out any visible thinning.

An ordinary coarse-grained silver bromoiodide in gelatin dispersion of the type used in radiography was chemically sensitized to its optimum speed. To this dis persion was added 30.7 mg. of bis(4-morpholinothiocarbonyl)disulfide per silver mole and 5.3 mg. of 1,3- diamino-S-methyl phenazinium chloride per silver mole.

The sealed turbine blades were then coated with the radiation-sensitive dispersion with the gelatin held at a temperature of C. After coating with polyvinyl alcochloride absent from the dispersions. Developed silver marks were observed over a much larger percentage of the surfaces of the test articles. This indicated that silver marks were being produced by b th the radioactive gas adsorbed within the surface adjacent voids and by surface released radiation.

A coarse-grained silver b omoiodide dispersion in gelatin of the type conventionally employed in radiography is chemically sensitized to its optimum speed. The dispersion is then divided into equal portions and desensitizer of the type indicated below was added. The dispersions were separately coated on a film support at a co erage of 86.11 mgldm Separate coated samples containing one of the desensitizers were then exposed to emissions from radioactive sources exhibiting energy levels of 80, 250 or 500 KeV, for stepped time intervals. The exposed samples were developed f r 6 minutes in Kodak Developer D -l9. The following table summarizes the observed result:

NA. Not applicable hol and silver halide dispersion the turbine blades were held in the dark for 4 hours. The blades were then, while still in the dark, immersed in a tank of a methyl-paminophenol sulfate-hydroquinone developer similar to Kodak Developer D-l9 at 20C for 4 minutes, rinsed in water, fixed in a fixing bath (commercially available under the trademark Kodak F-S) and washed.

The turbine blades were examined visually and surface adjacent voids were indicated on some of the blades by developed silver marks on the processed dispersion coating. These surface adjacent voids were confirmed by sawing through the turbine blade beside the surface adjacent void. The above procedure was repeated omitting the polyvinyl alcohol sealant layer, but identically chill setting the silver bromoiodide in gelatin dispersion in its place. A similar methyl-p-aminophenol sulfate-hydroquinone developer (commercially available as under the trademark Kodak Developer D-l9) was used. Substantially uniform coatings were again formed with similar effectiveness in detection of surface adjacent voids.

When the dispersion was modified by the inclusion of 0.2 percent by weight titanium dioxide, the silver marks were more easily seen because of the white background provided by the pigment. The inclusion of 1 percent by weight hydroxyethylceilulose to the dispersion rendered it more easily spread upon the article surface during hand coating.

Procedures similar to those described above were employed, but with the bis(4-morpholinothiocarbonylldisulfidc and 1,3-diamino-5-methyl phenazinium While this invention has been disclosed with reference to certain preferred embodiments, it is recognized that numerous variations will readily occur to those having ordinary skill in the art. It is accordingly intended that the scope of this invention be determined by reference to the following claims.

We claim:

1. A method of nondestructively testing an article for substantially invisible voids adjacent its surface comprising:

a. bringing a radioactive gas into contact with the surface of the article being tested,

b. retaining by adsorption the radioactive gas on those surfaces presented by the substantially invisible voids and at other locations adjacent the surface,

c. juxtaposing a silver halide radiographic layer adjacent the surface being tested, and

d. preferentially recording within the silver halide radiographic layer the location of the radioactive gas adsorbed within the substantially invisible voids.

2. A method of nondestructively testing according to claim 1 in which a sealant composition is applied to the article surface to retain the adsorbed radioactive gas adjacent the substantially invisible voids.

3. A method of nondestructively testing according to claim 2 in which the silver halide radiographic layer forms the sealant composition.

4. A method of nondestructively testing according to claim 2 in which a separate, nonimaging sealant layer is 19 applied before the silver halide radiographic layer is juxtaposed adjacent the surface being tested.

5. A method of nondestructively testing according to claim 2 in which the sealant composition is applied as a uniform coating to the article surface by chill setting.

6. A method of nondestructively testing an article for substantially invisible voids adjacent its surface in which the article has on its surface contaminants providing sites for gas retention comprising:

a. bringing a radioactive gas capable of generating high energy emissions into contact with the surface of the article being tested.

b. retaining by adsorption the radioactive gas on those surfaces presented by the substantially invisible voids and at the surface retention sites,

c. juxtaposing a silver halide radiographic layer desensitized to high energy radiation adjacent the surface being tested, and

d. preferentially recording within the silver halide radiographic layer the location of the radioactive gas adsorbed within the substantially invisible voids.

7. A method of nondestructively testing according to claim 6 in which the silver halide is desensitized with at least one compound chosen from the class consisting of a piazine, thiuram disulfide, nitro-I ,2,3-benzothiazole, nitron, nitroindazole or S-mercapto tetrazole desensitizer.

8. A method of nondestructively testing according to claim 7 in which the silver halide is desensitized with a piazine and a thiuram disulfide in combination.

9. A method of nondestructively testing according to claim 7 in which the silver halide is desensitized with a S-nitroindazole.

10. A method of nondestructively testing according to claim 7 in which the silver halide is desensitized with a l-phenyl-S-mercapto tetrazole.

11. A method of nond structively testing according to claim 6 in which the silver halide is desensitized to room light.

12. A method of nondestructively testing according to claim 6 it which the silver halide is d sensit zed to high a d I) v energy ra ation.

13. A method of nondesrucfively t sting accordi g to claim 6 in wh ch the radioactive g s is krypton.

14. A method of nondes 'ctively testing an article for substan i lly invisible voids adj cent its s fa e compris ng:

a. bringing a radioactive gas which produces pre ominantly emissions of high energy into co-i' ct with the surf ce of the artic'e being tested,

b. retaining by adsorption the radioactive gas or these surfaces present d by th subst'--ntia.ly invisi- 20 ble voids and at other locations adjacent the surface,

c. juxtaposing adjacent the surface being tested a silver halide radiographic layer which is sensitive to X-ray radiation and which has been selectively desensitized to the high energy emissions,

(1. bombarding the surface of the substantially invisible voids with high energy radiation to stimulate X-ray emissions from these surfaces, and

e. preferentially recording within the silver halide radiographic layer the sites of X-ray radiation.

15. A method of nondestructively testing according to claim 14 in which the radioactive gas produces predominantly alpha or beta radiation.

16. A method of nondestructively testing according to claim 14 in which the silver halide radiographic layer contains a pigment chosen to provide a ready visual contrast with a silver image resulting from the substantially invisible voids.

17. A method of nondestructively testing according to claim 16 in which the silver halide radiographic layer contains a white pigment.

18. A method of nondestructively testing according to claim 17 in which the white pigment is titanium dioxide.

19. A method of nondestructively testing according to claim 14 in which the silver halide radiographic ayer contains a thickening agent.

20. A method of nondestructively testing an article for substantially invisible voids adjacent its surface compris ng:

a. bringing a radioactive krypton gas into contact with the surface of the article being tested,

b. sorbing the radioactive krypton gas onto all the surf ces presented by the article surface being tested,

c. allowing a portion of the radioactive krypton gas to di se from the surfaces of the article,

d. coating the surface of the artic e being tested with a silver h ide dispersion which is sens tive to X-ray radi t n and which has been desensit zed to high e'ier y bet". rqdtation and to room li ht and which conta s a tbickering agent and a whi e pigment,

e. allowing th co ting to remain on the a ticle surf ce "r' f'l'b 'l util a la nt im ge f t e SSlffiCC a 'f'cent voi s h s fo med rm, and

0: i1 1 -M pr ',r'r- ,;"i't9 t e fhmr rrli le (,iSPfiI'SQI' o" t;' .w to "o'ivet ti e l te t i".-f' a perm n nt, enlarg vis ble iri'ge of the s'" I "ly invi ible surf c adjacent Vt HS. 

1. A METHOD OF NONDESTRUCTIVELY TESTING AN ARTICLE FOR SUBSTANTIALLY INVISIBLE VOIDS ADJACENT ITS SURFACE COMPRISING: A. BRINGING A RADIOACTIVE GAS INTO CONTACT WITH THE SURFACE OF THE ARTICLE BEING TESTED, B. RETAINING BY ADSORPTION THE RADIOACTIVE GAS ON THOSE SURFACES PRESENTED BY THE SUBSTANTIALLY INVISIBLE VOIDS AND AT OTHER LOCATIONS ADJACENT THE SURFACE, C. JUXTAPOSING A SILVER HALIDE RADIOGRAPHIC LAYER ADJACENT THE SURFACE BEING TESTED, AND D. PREFERENTIALLY RECORDING WITHIN THE SILVER HALIDE RADIOGRAPHIC LAYER THE LOCATION OF THE RADIOACTIVE GAS ADSORBED WITHIN THE SUBSTANTIALLY INVISIBLE VOIDS.
 2. A method of nondestructively testing according to claim 1 in which a sealant composition is applied to the article surface to retain the adsorbed radioactive gas adjacent the substantially invisible voids.
 3. A method of nondestructively testing according to claim 2 in which the silver halide radiographic layer forms the sealant composition.
 4. A method of nondestructively testing according to claim 2 in which a separate, nonimaging sealant layer is applied before the silver halide radiographic layer is juxtaposed adjacent the surface being tested.
 5. A method of nondestructively testing according to claim 2 in which the sealant composition is applied as a uniform coating to the article surface by chill setting.
 6. A method of nondestructively testing an article for substantially invisible voids adjacent its surface in which the article has on its surface contaminants providing sites for gas retention comprising: a. bringing a radioactive gas capable of generating high energy emissions into contact with the surface of the article being tested, b. retaining by adsorption the radioactive gas on those surfaces presented by the substantially invisible voids and at the surface retention sites, c. juxtaposing a silver halide radiographic layer desensitized to high energy radiation adjacent the surface being tested, and d. preferenTially recording within the silver halide radiographic layer the location of the radioactive gas adsorbed within the substantially invisible voids.
 7. A method of nondestructively testing according to claim 6 in which the silver halide is desensitized with at least one compound chosen from the class consisting of a piazine, thiuram disulfide, nitro-1,2,3-benzothiazole, nitron, nitroindazole or 5-mercapto tetrazole desensitizer.
 8. A method of nondestructively testing according to claim 7 in which the silver halide is desensitized with a piazine and a thiuram disulfide in combination.
 9. A method of nondestructively testing according to claim 7 in which the silver halide is desensitized with a 5-nitroindazole.
 10. A method of nondestructively testing according to claim 7 in which the silver halide is desensitized with a 1-phenyl-5-mercapto tetrazole.
 11. A method of nondestructively testing according to claim 6 in which the silver halide is desensitized to room light.
 12. A method of nondestructively testing according to claim 6 in which the silver halide is desensitized to high and low energy radiation.
 13. A method of nondestructively testing according to claim 6 in which the radioactive gas is krypton.
 14. A method of nondestructively testing an article for substantially invisible voids adjacent its surface comprising: a. bringing a radioactive gas which produces predominantly emissions of high energy into contact with the surface of the article being tested, b. retaining by adsorption the radioactive gas on those surfaces presented by the substantially invisible voids and at other locations adjacent the surface, c. juxtaposing adjacent the surface being tested a silver halide radiographic layer which is sensitive to X-ray radiation and which has been selectively desensitized to the high energy emissions, d. bombarding the surface of the substantially invisible voids with high energy radiation to stimulate X-ray emissions from these surfaces, and e. preferentially recording within the silver halide radiographic layer the sites of X-ray radiation.
 15. A method of nondestructively testing according to claim 14 in which the radioactive gas produces predominantly alpha or beta radiation.
 16. A method of nondestructively testing according to claim 14 in which the silver halide radiographic layer contains a pigment chosen to provide a ready visual contrast with a silver image resulting from the substantially invisible voids.
 17. A method of nondestructively testing according to claim 16 in which the silver halide radiographic layer contains a white pigment.
 18. A method of nondestructively testing according to claim 17 in which the white pigment is titanium dioxide.
 19. A method of nondestructively testing according to claim 14 in which the silver halide radiographic layer contains a thickening agent.
 20. A method of nondestructively testing an article for substantially invisible voids adjacent its surface comprising: a. bringing a radioactive krypton gas into contact with the surface of the article being tested, b. sorbing the radioactive krypton gas onto all the surfaces presented by the article surface being tested, c. allowing a portion of the radioactive krypton gas to diffuse from the surfaces of the article, d. coating the surface of the article being tested with a silver halide dispersion which is sensitive to X-ray radiation and which has been desensitized to high energy beta radiation and to room light and which contains a thickening agent and a white pigment, e. allowing the coating to remain on the article surface undisturbed until a latent image of the surface adjacent voids has formed therein, and f. thereafter processing the silver halide dispersion coating to convert the latent image to a permanent, enlarged visible image of the substantially invisible surface adjacent voids. 